When a first communication line is abnormal, one of first ECUs as a master node transmits a reference signal to a second communication line. Upon receiving the reference signal, the first ECUs transmit synchronous data items. The reference signal has a higher priority than an asynchronous data item; the synchronous data items each have a higher priority than the asynchronous data item. The first ECUs performs transmissions of entire synchronous data items needed to be transmitted during one transmission cycle of the reference signal at mutual different points of time so that the asynchronous data item is permitted to be transmitted during an interval between the transmissions of the synchronous data items.